Task analysis is meant to identify user goals and tasks when using an interactive system. In the case of users performing real-life work, task analysis can be a cumbersome process gathering a huge amount of unorganized information. Task Models provide a mean for the analyst to organize information gathered during task analysis in an abstract way that can be further detailed if needed. This chapter aims at providing newcomers with background in task modeling. It provides an overview on how the recent advances in task description techniques can be exploited to design and assess interactive systems. As task models can be large it is important to provide the analyst with computer-based tools for editing task models, for analyzing them and for simulating them. For that, the HAMSTERS notation and its eponym modeling tool are used in this chapter to illustrate the examples issued from real industrial projects.